This invention relates to a novel golf ball having an improved cover composition, which cover composition imparts superior properties to the golf balls. More particularly, this invention relates to a novel golf ball having an improved cover composition such that the resulting golf ball exhibits improved initial velocity, superior aging characteristics, improved coefficient of restitution, and better low temperature impact properties.
Golf balls having cover compositions which generally comprise a copolymer of an olefin and at least one unsaturated carboxylic acid are known in the prior art. These cover material compositions are sold under the trademark "Surlyn" by the E. I. du Pont de Nemours Company, Wilmington, Del. The cover compositions are highly advantageous in that the resulting covers are extremely resistant to cuts and abrasion. These balls are commonly referred to in golfing circles as cutless balls. These prior art golf balls are generally described in U.S. Pat. No. 3,454,280, issued on Jul. 8, 1969.
While being extremely advantageous from the standpoint of cut resistance, the golf balls as described in U.S. Pat. No. 3,454,280 have a shortcoming in that they do not have cold temperature cracking properties such as to permit their use at temperatures below freezing. In fact, when the balls of the prior art are played at temperatures below 32.degree. F., they have a tendency to crack or explode when struck by a golf club. Likewise, the balls of this patent are lacking in their coefficient of restitution, a property related to the distance which a golf ball can be driven.
One attempt to overcome the problems of these prior art golf balls is described in U.S. Pat. No. 3,819,768, issued Jun. 25, 1974. This patent describes golf ball covers made of compositions comprising at least two different ionic copolymers which are produced by the reaction of an olefin having from two to five carbon atoms with a metal salt of an unsaturated carboxylic acid containing from three to eight carbon atoms. In particular, the disclosed cover compositions of that patent comprise mixtures of a copolymer of an olefin and a sodium salt of an unsaturated monocarboxylic acid and a copolymer of an olefin and a zinc salt of an unsaturated monocarboxylic acid. This patent is owned by Questor Corp., manufacturer of golf balls sold under the trademark TopFlite. While the Questor patent claims that all commercially available blends of ionomer resins of different metal ions, i.e., all blends of zinc ionomer resins and sodium ionomer resins, will have the improved coefficient of restitution, it has been found that most blends of these two resins, other than the Surlyn 1555 and Surlyn 1557 as specifically set forth in the Questor patent, do not have an improved coefficient of restitution. It has also been discovered that these blends are generally cut when subjected to the so-called guillotine test.
Another attempt to overcome the problems of the prior art golf ball cover compositions is disclosed in U.S. Pat. No. 4,323,247 issued Apr. 6, 1982. That patent discloses a triblend composition comprising two sodium-based Surlyn resins and a zinc-based Surlyn resin, wherein each of the three resins has specific limitations on the percentage of free acid, the percentage of total acid, the percentage of metal ion content, and the average melt flow. While this patent discloses golf balls of improved coefficient of restitution and initial velocity, it does not disclose improved cold temperature impact properties or aging properties.